Curious what you're looking for in a path+querystring builder.

For ASP.NET controllers, I'd use attribute routing's URL generation: https://docs.microsoft.com/en-us/aspnet/core/mvc/controllers...

If you want a separate library for use outside of ASP.NET, and you want something more sophisticated than UriBuilder, there are several out there. I've used Superscribe in the past, but I just looked around and Flurl looks pretty neat: http://tmenier.github.io/Flurl/